Various instruments are known for the analysis of blood gases, these instruments ranging from ordinary routine apparatus to the most complex automated apparatus, which are used for the real-time monitoring of the levels of pO.sub.2 and pCO.sub.2 in the blood, along with other components, during surgical operations or intensive care.
These instruments, which operate at 37.degree. C., comprise a series of specific electrochemical sensors for each individual determination, the sensors requiring frequent, if not continuous, calibration.
The term "electrochemical sensors" is understood to refer to potentiometric, amperometric or conductimetric systems which measure electroactive species in solution, either directly (electrolytic species) or indirectly (gases), by means of membranes which are selective for ions or gases respectively.
The ion-selective electrodes are calibrated using reference solutions, whereas the gas-selective electrodes may be calibrated using standardized gas mixtures, with ambient air or with reference liquids, the latter consisting of solutions equilibrated with gas mixtures at constant temperature.
The calibration of the gas-selective electrodes, in particular of the oxygen electrode used for the determination of the pO.sub.2 in the blood, is of particular interest and many methods have been developed to carry out such a calibration.
The determination of the partial pressure of oxygen using instruments for the analysis of blood gases, which is carried out with a Clark amperometric electrode with which the pO.sub.2 is determined in millimetres of mercury (mmHg), is known in the art.
In this case, calibration of the system takes place by recording the current signal of the oxygensensitive electrode at two known values of pO.sub.2.
The first calibration point has a zero pO.sup.2 value and is used to determine a base value.
The second calibration point, on the other hand, has a known pO.sub.2 value which varies according to the type of instrument used and is between 100 and 200 mmHg.
This second calibration point serves to determine the slope of the calibration curve.
The first calibration point may be established in one of the following ways:
1) with a mixture of gases contained in a suitable cylinder having a zero concentration of oxygen; PA1 2) with an aqueous solution containing a strongly reducing substance (sodium sulphite, sodium dithionite, etc.), in which the oxygen is chemically consumed; PA1 3) using electrochemical methods which allow the oxygen to be removed by electrolysis; PA1 4) by carrying out electrical zeroing on the instrument. PA1 a) from a mixture of gases contained in a suitable cylinder with a known concentration of oxygen; by knowing the atmospheric pressure at which the measurement is taken, it is possible to obtain a known value of pO.sub.2 ; PA1 b) from atmospheric air in which the percentage of oxygen is constant (20.9%); in this case, also, it is necessary to know the atmospheric pressure when the measurement is taken; PA1 c) with an aqueous solution equilibrated, when it is produced, with a gas mixture containing a suitable percentage of oxygen and packaged in collapsible, gas-impermeable containers; PA1 d) with a solution equilibrated with atmospheric air at the same temperature as the measuring temperature (37.degree. C.), with a stage of tonometry carried out directly on the instrument by bubbling or by passage through plastic, oxygen-permeable tubes or membranes; in this case, also, it is necessary to know the atmospheric pressure when the measurement is taken. PA1 providing available at least one buffered calibration solution containing O.sub.2 and a tonometer-measured or known amount of CO.sub.2, the said solution moreover comprising known concentrations of the electrolytes and metabolites to be determined; PA1 determining the pO.sub.2 titre in at least one said calibration solution; PA1 calibrating the electrodes in a single cycle with at least one said calibration solution in which the pO.sub.2 titre has been determined;
On the other hand, the second calibration point may be established in one of the following ways:
The methods listed above for establishing the second calibration point each have advantages and disadvantages.
Indeed, method a) has the advantage of being reliable, since a mixture of gases is used which is a standard controlled by extremely accurate gas-chromatographic reference methods.
Moreover, this allows the simultaneous calibration of the pO.sub.2 and pCO.sub.2 values.
However, this method has drawbacks associated with the use of compressed-gas cylinders, namely their bulk, safety problems and the difficulties involved in moving the instrument around.
Moreover, since the instruments also measure other parameters which require agents for calibrating in aqueous solution, it is necessary to carry out mixed cycles of calibration, partly with a gas mixture and partly with aqueous solutions, thereby considerably lengthening the time required to calibrate the instrument and/or complicating the fluid route.
Method b) for establishing the second calibration point has the advantage of using ambient air, which does not cost anything and has a constant composition, irrespective of location, thereby making it, de facto, a primary standard.
However, with ambient air, it is not possible to carry out the calibration for the determination of the pCO.sub.2 and therefore instruments which use this method are coupled to a cylinder of carbon dioxide.
A variant of an instrument which uses the abovementioned method b) is the so-called Gas Mixer, in which mixtures of gases from air and from a cylinder of carbon dioxide are obtained directly from the instrument.
Lastly, method b) also necessarily uses mixed calibration cycles.
Method c) has the advantage over the preceding methods of being able to calibrate electrodes for gases and for electrolytes simultaneously, thus using shorter cycles without complicating the fluid route.
Moreover, the instrumentation is of minimal bulk since it does not require a cylinder, and the safety problems associated with the use of gas mixtures are also avoided.
However, this method suffers from the drawbacks associated with the use of tonometer-measured solutions.
Such solutions contain known dissolved amounts of oxygen and carbon dioxide together with known amounts of other components to be determined, such as bicarbonate, calcium, sodium, potassium, other ionic species and organic species such as glucose.
These solutions may be prepared in a tonometer by mixing the appropriate ingredients together so as to obtain a buffered solution and adding to this solution a gas mixture with a known titre of oxygen and carbon dioxide.
The gases and the aqueous phase equilibrate in the tonometer and the partial pressures of oxygen and of carbon dioxide are adjusted to the desired values.
These values correspond to molar concentrations which are considerably lower than the concentrations of these gases in the gas mixtures.
At this point, the solution thus prepared is stored in a gas-impermeable, collapsible sealed container which is generally a bag consisting of an aluminium laminate placed between layers of a thermoplastic polymer.
The solution may be sealed inside this bag at a gas pressure below atmospheric pressure, as described, for example, in U.S. Pat. No. 4,116,336, or above atmospheric pressure by also introducing a gas which diffuses rapidly, such as helium, into the bag, as described in U.S. Pat. No. 4,960,708.
In any case, calibration using these tonometer-measured solutions is almost always inaccurate as far as determining the pO.sub.2 is concerned, since it has been found that the content of this gas in the bags tends to vary between the time they are prepared and the time they are used. The reason for this is that the metal layers of the bags are never totally impermeable to gases, especially to oxygen.
Moreover, the permeability of the gases may increase considerably, for example because the packaging is not completely leaktight and/or because of the possible exposure of the bags to high temperatures, with consequent degassing of the solution.
However, the abovementioned drawbacks essentially relate to variations in the pO.sub.2 titre in the bag, since the pCO.sub.2 titre tends to remain stable over time because of the intrinsic properties of the formulations of the solutions contained in the bags.
Moreover, the pO.sub.2 titre in the bag may also decrease as a result of corrosion phenomena occurring in the metal layer of the containers with which the solution may come into contact during storage and/or because of microbiological contamination.
Consequently, the shelf-life of these solutions is also relatively short.
Method d) has essentially the same advantages as method c).
However, in this case also, the pO.sub.2 titre of the solutions used is quite variable.
Moreover, the solution which is tonometer-measured with air cannot be used to calibrate the pCO.sub.2 and the related instrumentation is quite complex and expensive, since it requires a thermostatically-controlled tonometry system.
Consequently, the calibration cycle is not the same for oxygen and the other parameters.